The invention concerns a speed-control system for a brushless direct-current motor using a digital servo loop, specifically an economical and simple means of generating precise information relating to speed of rotation that can also be converted into digital words that can be processed by a microprocessor or by wired components in normal digital switching circuits like counters, gates, adders and subtractors, flip-flops, multipliers (such as arithmetic and logic circuits), memories, etc.
The present invention is applicable to brushless motors of this type that have a speed higher than 10 rpm, like those employed to operate turntables for normal and digital record players, and audio or video tape recorders.
Speed-control systems for brushless direct-current motor can, as is known, utilize a number of devices to supply speed information to their shaft. An example of such a device is a light barrier with a source of infrared radiation that emits a slender beam of light, with a detector for the radiation positioned where it can receive the beam either directly or reflected from a mirror, and with a rotating component that extends into the path of the light and affects its transmission to the detector. The rotating component can be a perforated disk mounted on the shaft of the motor that deflects the beam to the detector every time its shaft rotates once. The detector can be a phototransistor or photodiode that releases one pulse per rotation. The disk can be replaced with a rotating finger that interrupts the beam every time the shaft rotates once. Another way of employing a light barrier is to mount a rotating mirror on a component driven by the shaft and to position a light source and a detector in such a way that the beam of light will strike the surface of the detector once per revolution. The interval of time between the beginning or end of two subsequent pulses emitted by the detector can be measured and the result of the measurement be compared with a stored digital value. The difference is employed as an error signal in a regulating loop in such a way that the operating voltage at the stator windings is varied in order to minimize the difference.
Another known way of generating a signal that contains information relating to the speed of rotation of the shaft of a motor consists of using a tachometer-generator of the type that has a peripheral annular magnet, preferably surrounding the permanent-magnet rotor, with a number of poles that is a multiple of the number of rotor poles, and with a meandering circular or annular winding facing the outside of the ring and mounted on the substrate of a printed circuit. The signal induced in this tachometer winding has an amplitude and frequency proportional to the speed of the motor. The frequency of the signal can be utilized to derive a digital value employed in the control loop.
Both of the aforementioned means of generating speed information demand additional mechanical, opto-electrical, or magnetic components, and the latter entails an increase in the height of the axial air gap between the permanent-magnet rotor and the stator-drive coils. This decreases the motor torque. Furthermore, the induced-signal amplitude in the meander winding is relatively small (on the order of 5 mV) and must accordingly be amplified, filtered, and limited (sliced) to obtained a practical rectangular signal for a digital switching circuit or for microprocessors. All of this leads to increased expense. A circuit that employs the output voltage of a tachometer-generator is disclosed in FIG. 9 of U.S. Pat. No. 4,394,594 or GB Patent No. 1 563 228 or the corresponding German OS No. 2 533 187.
Another method of controlling the speed of a brushless direct-current motor with a permanent-magnet rotor is specified in U.S. Pat. No. 3,924,166 or French Patent Application No. 2 204 073, corresponding to German OS P No. 2 251 292 (FIG. 1). This method employs the opposing electromagnetic force (emf) induced in the drive coils that constitute the stator windings while switching transistors downstream of the direct-current source are turned off. The opposing emf has a polarity that is opposite that of the voltage at the winding as long as the transistors are connected through and has the shape of half a sine wave with an amplitude proportional to the speed of the motor. The opposing emf's of all the windings are supplied through appropriate half-wave rectifier diodes to an adder network consisting of variable resistors, and the resulting current from this matrix-like addition is supplied along with a current from a constant-current source to the inverting input terminal of an operational amplifier. The other, non-inverting input terminal of the amplifier is connected to the terminal of a reference-voltage source that supplies a current corresponding to the desired speed. The output terminal of the operational amplifier is connected to the input electrode of a variable source of constant current interposed between one terminal of the source of voltage supply and the junction beteen all the emitters of the switching transistors of which the collectors constitute the load on the stator windings. This control loop, which employs the analog information from the opposing emf generated in all the windings, functions by varying the voltage at the windings when its corresponding series transistors are connected through, whereby the motor speed is governed.